1. Field of the Invention
The invention relates to the field of wavelength division multiplex (WDM) optical ring communication networks and more particularly to protecting the transmission of signals in such networks in the event of interruption of transmission.
2. Description of the Prior Art
Several solutions have been proposed to enable transmission of signals to continue in an optical ring network in the event of a problem affecting the transmission optical fibers or other transmission components.
The “1+1 O-SNCP” solution was developed for WDM networks. It consists in providing first and second pairs of monodirectional optical fibers and transmitting the same multiplexed signals in opposite directions (called “East” and “West”) in the first pair of fibers and in the second pair of fibers. The network equipments at the ends of connections defined by the first and second optical fiber pairs concern themselves only with signals that reach them from one of the two directions, for example the East direction. If one of these equipments detects an interruption in transmission of the signal in the East direction, it changes direction and no longer concerns itself with signals other than signals that reach it from the West direction.
A solution of this kind is very secure but is particularly costly because it entails doubling the number of transponders in each network equipment and uses at least twice the bandwidth.
The Shared Protection Ring (SPRing) solution was developed for SDH networks. A Multiplex Section SPRing (MS-SPRing) version dedicated to the transmission of multiplexed signals is defined by ITU-T standard G.841. It consists in connecting the network equipments two by two using two monodirectional optical fibers dedicated to transporting time division multiplexed signals. In normal operation, the signals circulate in the optical fibers at a first wavelength. If one of the equipments detects an interruption of signal transmission in the optical fiber sections to which it is connected, it integrates warning bytes conforming to the “K-byte” protocol into the header of the SDH signals (or messages) that contain the data. These bytes request the equipment at the opposite end of the connection to transmit its signals on the optical fibers in a direction opposite to the previous direction, by reconfiguring its transponders.
It is difficult to apply this mode of operation to WDM networks and more particularly to WDM metropolitan area networks in which signals of different types coexist and do not always have a header. It would certainly be possible to use a dedicated Optical Supervisory Channel (OSC) to enable the equipments to exchange frames, equivalent to SDH headers (32 bytes), comprising warning messages. However, this would monopolize transmission resources of the network and would entail developing new transponders, which could prove time-consuming and costly.
What is more, this second solution uses a relatively slow protection mechanism since it necessitates sending information back from one end to the other of each faulty connection.
There being no entirely satisfactory prior art solution, an object of the invention is therefore to improve on the situation.